1. Field of the Invention
The present invention is related to pressure regulators for breathing apparatus and, more particularly, to the regulator second stage of a self-contained underwater breathing apparatus (scuba).
2. Description of the Prior Art
The functional cycling of a scuba regulator is controlled by the respiratory effort of the diver. An ideal regulator requires very little effort to exactly provide for the diver's respiratory needs during any combination of work load and depth. However, various characteristics of the prior art prevent achievement of this ideal.
In a typical scuba, air or other breathable gas (hereinafter "air", "breathable gas" and "gas" will be used interchangeably) is supplied to a diver from a high pressure tank via a two stage pressure regulator. Typically, a filled high pressure tank holds air at a pressure in excess of 2,000 psi. The regulator first stage is connected to the tank valve and functions to reduce tank pressure to an intermediate pressure which is about 150 psi above ambient. The tank and first stage are, generally, carried as a unit on the diver's back. A flexible hose conduit conveys the intermediate pressure air from the regulator first stage to a regulator second stage. The regulator second stage opens into and is held by the diver's mouth.
Within a typical second stage, a normally closed valve is mechanically levered open to provide air flow when a diaphragm, which is exposed to ambient pressure, is pulled inward by the suction created as a result of the diver's inhalation effort. Whenever the diver stops breathing, or exhales, the diaphragm responds to the lack of inhalation induced suction by returning to its neutral position, thereby stopping the flow of gas. An exhaust valve is provided to permit exhaled gases to escape to ambient.
Diaphragms and exhaust valves must be designed to avoid detrimental interaction. A discussion of diaphragm and exhaust valve designs, and an inventive diaphragm and exhaust valve combination are disclosed in the inventor's U.S. Pat. No. 4,574,797 entitled Diaphragm and Exhaust Valve for Second Stage Regulators, issued Mar. 11, 1986. Another diaphragm and exhaust valve combination are disclosed in the inventor's U.S. Pat. No. RE 31,932 entitled Diaphragm Assembly for the Demand Regulator of a Breathing Assembly, reissued July 2, 1985. These patents are incorporated herein by reference.
The most common second stage valve design of the prior art is spring loaded to keep an unbalanced, downstream, intermediate pressure valve normally closed. The spring is designed to oppose the intermediate pressure force trying to push aside the seat. In addition, the spring must provide an extra force against the seat to assure a gas tight seal when closed.
Combinations of deep diving and strenuous activity can cause respiratory demands which exceed the flow capabilities of a regulator. Larger valves provide greater flow capacity. But larger unbalanced valves require stronger springs which are difficult to operate with low respiratory effort.
In a less common second stage valve design, the normally closed valve is balanced. In a balanced valve, the pressure force is negated and therefore does not work to either open or close the valve. A balanced second stage valve does not require a spring force to overcome the intermediate pressure force. The spring provides only the force needed to close the valve and maintain a gas tight seal. As a result, balanced valves can be sized larger to deliver higher flow rates without the penalty of a stronger spring.
But unbalanced, downstream second stage valves also double in function as safety relief valves. A safety relief valve is needed in the event that a malfunctioning first stage over pressurizes the intermediate pressure hose conduit. Balanced valves cannot function as safety relief valves. Consequently, an independent safety relief valve must be included in parallel with a second stage balanced valve.
Another variation of the balanced valve, which satisfies the need for a safety relief valve, is the semi-balanced, second stage valve. With this design, the valve is partially unbalanced just enough to open in the event of excessive intermediate pressure. The spring, consequently, must be increased in strength to compensate for the partial unbalance. As a compromise design for improved regulator performance, the semi-balanced valve retains the advantages of a balanced valve and avoids the need for an independent safety relief valve.
A number of inventive valve designs have been proposed in the prior art to improve the performance of scuba regulators. One such design is characterized by having a small mechanically levered pilot valve which controls the movement of a balanced, pressure assisted main valve. In this arrangement, the pilot valve will respond to very low inhalation effort. Consequently, the main valve, being power assisted by gas pressure, can be sized as large as desired. The inventor's U.S. Pat. No. 3,783,891 entitled Balanced Regulator Second Stage, issued Jan. 8, 1974; U.S. Pat. No. 4,076,041 entitled Pilot Valve Operated Demand Regulator for a Breathing Apparatus, issued Feb. 28, 1978; and U.S. Pat. No. 4,297,998 entitled Pilot Controlled Regulator Second Stage, issued Nov. 3, 1981 all disclose second stage valve mechanisms which utilize a pilot valve to control the movement of a pressure assisted main valve. Pilot and main valve designs significantly improve scuba regulator performance, but have proven costly to manufacture.
Another valve design is disclosed in U.S. Pat. No. 4,041,978 entitled Pressure Regulator for Breathing Apparatus, issued to Karl Leemann on Aug. 16, 1977. The Leemann regulator is a balanced valve with a venturi-like modification to the valve seat which assists opening in direct proportion to flow. The Leemann regulator suffers from unstable and low flow performance.
Yet another valve design is disclosed in U.S. Pat. No. 4,266,538 entitled Pressure Regulator, issued to Heinz Ruchti on May 12, 1981. The Ruchti regulator is an unbalanced valve which uses an adjustable linkage of high mechanical advantage to communicate movement of the diaphragm to the valve. High mechanical advantage is a desirable feature because less respiratory effort is required to operate the valve mechanism against a given spring load. However, the high mechanical advantage linkage in the Ruchti device has an undesirable shortened valve stroke which limits the distance the valve can open and, consequently, severely limits high flow performance.
All of the second stage designs of the prior art are subject to performance degradation due to mechanical friction. For example, friction occurs with sliding and rotary contact of the lever or linkage which communicates movement of the diaphragm to the second stage valve. Also, second stage valve seat assemblies and accompanying springs of the prior art typically rub against the valve housing during operation. These frictional forces must be overcome by respiratory effort and account for much of the effort needed to initiate flow.
During a respiratory cycle, the second stage valve must continuosly adjust output because flow into the lungs increases from zero (at the beginning of inhalation) to a maximum approximately half way into the breath, and back to zero as inhalation is completed. The transition from zero to maximum and back to zero flow should be smooth and uninterrupted. As the flow varies, frictional forces cause uneven or erratic operation of the regulator. Frictional forces also cause the valve to lag behind the actual demand, producing a hysteresis effect.